Written by: Austin Meadows
Updated March 4, 2021
Circadian rhythms refer to the natural fluctuations of bodily processes in plants, animals, and microbes along a 24-hour cycle. The phrase gets its name from the Latin "circa diem," (1) meaning "around a day." This phrase describes the way these cycles follow the sun and adhere to a relatively consistent day-night pattern.
Cells throughout the body are programmed to follow a roughly 24-hour cycle, causing body temperature, appetite, and energy levels to rise and fall at consistent times of the day. For humans, one of the most significant circadian rhythms is the sleep-wake cycle.
The genes and proteins that direct cells involved in the circadian process are referred to as biological clocks. Biological clocks are also responsible for other rhythms, such as what time of year a flower blooms (2).
How do Circadian Rhythms Work?
Circadian rhythms are coordinated by a master clock in the brain called the suprachiasmatic nucleus (SCN) (3). Located in the hypothalamus, the SCN is a cluster of approximately 20,000 nerve cells. The SCN sends out commands directing the body to activate certain pathways based on the time of day.
Many cells in the body also have their own intrinsic rhythms (4). The SCN's job is to synchronize these rhythms so that we don't wake up in the middle of the night wanting to eat, or experience a sudden lapse in energy just as dawn is breaking. Over the course of the day, circadian rhythm fluctuations (5) in appetite, energy expenditure, and other processes keep the body running efficiently (6).
How Do Circadian Rhythms Affect Sleep?
One of the most important functions of the circadian rhythm is to regulate when we sleep and when we wake. The SCN relies on light to direct the sleep-wake cycle. Faced with daylight, we show increased levels of alertness and energy. When it becomes dark, the SCN stimulates the release of a hormone called melatonin that prepares the body for sleep.
Most people experience predictable peaks and dips in alertness throughout the day. We tend to feel most awake in the morning and then experience an afternoon slump. Energy levels spike (7) again in the evening, then reach an all-time low in the middle of the night.
What Can Cause Circadian Rhythms To Become Out of Sync?
When the SCN receives contradictory information, sleep rhythms can become desynchronized from day and night.
A disrupted circadian rhythm is common in shift work, in which people follow an irregular sleep-wake cycle that may require staying awake at night or sleeping during the day. A disrupted circadian rhythm is also the cause of jet lag, which is when an individual's body clock is not yet synchronized with the new local time. When these schedules interfere significantly with a person's circadian rhythm, the person is considered to have an external circadian rhythm disorder.
Certain medications, health conditions, and habits, such as using screens late at night, can also interfere with natural sleep cycles.
What is an Intrinsic Circadian Rhythm Sleep Disorder?
People with intrinsic circadian rhythm sleep disorders have an internal clock that doesn't fit the standard day-night model. Two examples of such disorders are advanced sleep-wake phase disorder and delayed sleep-wake phase disorder.
Other circadian rhythm disorders include non-24-hour sleep-wake disorder, in which the circadian rhythm is longer than 24 hours and causes progressively later bedtimes, and irregular sleep wake disorder, which is when an individual lacks a consolidated sleep period. Irregular sleep-wake disorder is most common in people with neurodegenerative diseases (8).
What Happens When Your Circadian Rhythm is Out of Sync?
The most obvious symptoms of your circadian rhythm being desynchronized are an inability to sleep when you should, or intense feelings of sleepiness when you are meant to be awake. Even if you do manage to fall asleep at night, you might not sleep as soundly and you could be easily awakened by external cues.
It's also possible for different circadian rhythms within your body to fall out of sync with each other. This desynchronization among circadian rhythms can interfere with digestion, blood pressure, and other processes, and it likely contributes to the link between circadian rhythm problems and metabolic disorders (9).
Over the long run, disruptions to the circadian rhythm have been linked to a host of health problems, including obesity, diabetes, heart disease, and mood disorders (10).
Can You Change Your Circadian Rhythm?
Most people have a natural inclination to wake up at a certain time, a concept that's known as chronotype (11). Morning-type chronotypes have an internal alarm clock that makes it difficult to sleep in no matter what they do, while evening-type chronotypes may have a tough time going to bed early. Chronotype shifts with age and varies from person to person.
Although chronotype is largely genetic, there are some lifestyle habits that may help align your circadian rhythm to a more convenient schedule.
How Do I Regulate My Circadian Rhythm?
You can entrain your circadian clock to a certain extent by consistently providing your body with certain cues at the right times. These signals help train your body to recognize when you would like to sleep and when you would prefer to be awake. Some strategies include:
- Setting your alarm at the same time every day
- Receiving exposure to bright light (12) soon after you wake up
- Eating a healthy diet and avoiding large meals at night
- Exercising regularly
- Limiting naps, especially late in the day
- Avoiding caffeine, alcohol, and tobacco in the evening
- Turning off screens at least one hour before bed
- Reserving the bedroom for sleep and sex only
- Keeping the bedroom cool, dark, and quiet
Everyone reacts differently to these cues, so you may need to experiment and see what works for you. Talk to your doctor if your sleep clock is still acting up after trying these sleep hygiene tips. A doctor can help pinpoint the source of any sleep problems and offer additional treatment options to help you sleep better.
- https://pubmed.ncbi.nlm.nih.gov/31756974/ Accessed on February 18, 2021.
- https://www.nigms.nih.gov/education/Inside-Life-Science/Pages/tick-tock-new-clues-about-biological-clocks-and-health.aspx Accessed on February 18, 2021.
- https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx Accessed on February 18, 2021.
- https://pubmed.ncbi.nlm.nih.gov/23385698/ Accessed on February 18, 2021.
- https://pubmed.ncbi.nlm.nih.gov/21633182/ Accessed on February 18, 2021.
- https://www.sciencedirect.com/science/article/pii/S0074774216301167 Accessed on February 18, 2021.
- https://pubmed.ncbi.nlm.nih.gov/23585751/ Accessed on February 18, 2021.
- https://pubmed.ncbi.nlm.nih.gov/20631712/ Accessed on February 18, 2021.
- https://pubmed.ncbi.nlm.nih.gov/27763782/ Accessed on February 18, 2021.
- https://pubmed.ncbi.nlm.nih.gov/29230328/ Accessed on February 18, 2021.
- https://pubmed.ncbi.nlm.nih.gov/28636610/ Accessed on February 18, 2021.
- https://pubmed.ncbi.nlm.nih.gov/30311830/ Accessed on February 18, 2021.